The Vernier machine (VM) was first introduced as a synchronous reluctance motor variant less than 60 years ago and a permanent magnet version of this type of machine appeared only a little more than 20 years ago. In this type of machine, the rotor rotates relatively slowly, and only at a definite fraction of the angular velocity of the stator rotating field. Meanwhile, the motor torque steps up as the rotor speed steps down with this type of operation. This so-called electric gearing effect makes the VM an attractive alternative for direct-drive applications and is especially suitable for low speed motoring/braking operation. However, current designs of the Vernier permanent magnet machine (VPPM) have low power factor because the permanent magnets added to the rotor create additional harmonic flux leakage in the air gap.
To further increase torque density and cope with the low power factor feature of VPPMs, researchers have proposed two major designs of double air gap VPPMs in terms of machine structure: 1) a double rotor Vernier permanent magnet machine (DRVPPM), and 2) a dual stator Vernier permanent magnet machine (DSVPPM). It has been shown that torque improvement and copper loss reduction are achieved by the adaption of the double rotor structure, while the DSVPPM further improves both torque density and power factor with a dual stator spoke array VPPM topology. Unlike conventional single stator and rotor topology, however, introduction of an additional layer of rotor in DRVPPM or an additional layer of stator in DSVPPM significantly increases the machine manufacturing complexity, and poses a challenge to the mechanical structural integrity as well. A thermal issue becomes another concern because the inner stator is enclosed by rotating parts which limit the cooling options for windings positioned in that area.